For the last 40 years 5-fluorouracil (5-FU) has been the most active chemotherapy agent for colorectal cancer (CRC). Despite the advent of newer agents (oxaliplatin, irinotecan (CPT-11), clinical studies have demonstrated that significant improvement in response rates and median survival are seen only when these agents are combined with 5-FU. Thus 5-FU-resistance in CRC cells will adversely affect the effectiveness of these promising combination regimens, and strategies to reverse 5-FU resistance may significantly improve treatment efficacy. Although a major mechanism of 5-FU resistance in CRC is through overexpression of the dTMP synthesizing enzyme thymidylate synthase (TS), approaches to overcome increased TS levels have been largely unexplored. We have found in vitro that low concentrations of arsenic trioxide (ATO) effectively inhibited TS protein and gene expression, and reversed 5-FU resistance in CRC cell lines. Furthermore, we have found that patients with metastatic CRC receiving ATO as a single agent had a significant (p=0.03) decrease in TS gene expression in correlative studies of their peripheral blood mononuclear cells (PBMC). Finally, initial results from the first two CRC patients treated with ATO + 5-FU in our phase I trial have demonstrated that the combination (at the doses given) had minimal toxicity, and resulted in downregulation of TS in both PBMC and in the tumor tissue itself. In this R21 application we are proposing a phase I clinical trial to determine the maximally tolerated dose of both ATO and 5-FU (in combination) in patients with metastatic CRC. The second objective is to determine whether ATO serum concentration correlates with the down regulation of thymidylate synthase expression in tumor tissue and PBMC. These findings should lead to establishment of a phase II dose of ATO + 5-FU that can be definitively tested for efficacy and evidence of reversal of 5-FU resistance, as well as providing a more conclusive correlation between ATO levels and TS downregulation - which may be useful in further modifying the treatment regimen. Aim 1: To perform a phase I study to determine a dose combination of 5-FU and ATO that can be safely administered for the treatment of 5-FU resistant relapsed/refractory colorectal cancer patients. Aim 2: Determine if ATO administration down regulates the expression of thymidylate synthase in tumor and in PBMC in patients receiving ATO + 5-FU. [unreadable] [unreadable] [unreadable]